How Many Various Gains brought by the MIMO for LTE
When we talk about LTE RF Planning then the one question in mind is that what about MIMO gains. Here I write Different Gain in LTE like MIMO Gains and Gains which brought by multiple antenna technology and By MIMO Like Power Combine Gain, Array Gain, Space Diversity Gain, Interference reduction gain and Spatial multiplexing gain.
MIMO configuration indicates that multiple antennas are used for signal transmission at the transmit end and signal reception at the receive end to improve the quality of service (QoS) for each subscriber. For a traditional single-antenna system, the MIMO technology can be classified into Single Input Multiple Output (SIMO) and Multiple Input Single Output (MISO) according to the number of antennas at the transmit and receive ends.
The figure below shows one example for Transmit Diversity system structure. For Spatial Multiplexing MIMO, CW0 and CW1 will be fed by 2 different payload streams.
In the example above, a single data input stream is used to feed two separate antennae but it is equally possible to feed two different input data streams into this setup to provide higher data rate and it is the fundamental principle for Multiple Code Word (MCW) in LTE.
Multi-antenna technology can improve system capacity and coverage without largely increasing cost. This is because the multi-antenna technology gives the following gains: power combining gain, array gain, space diversity gain, and interference reduction gain. In addition, the MIMO achieves a spatial multiplexing gain, which is mainly used to increase the system capacity.
Power Combining Gain
When multiple antennae (N) are used to transmit signals, N transmit channels are available. In this case, the total transmit power is equal to N times the transmit power from a single antenna signal transmission. As a result, a power gain of 10log(N) dB can be obtained. If a single antenna is used to transmit signals, you can also increase the transmit power. In this case, however, the requirements for the power amplifier are high, and implementation cost is complex and increased.
The array gain indicates an improvement in the average signal noise ratio (SINR) at the receive end when the total transmit power is the same. The array gain can be obtained through the coherent combining of various antenna signals. Various multi-antenna systems can obtain the array gain. That is, after the multi-antenna technology is used, the receiving SINR can be improved.
Space Diversity Gain
Due to the fading nature of wireless channels, the signals in a single-antenna system suffer from deep fading. In a multi-antenna system, the distance between antennae is often large. This ensures that the signal fading of an antenna is independent. Therefore, the SINR fluctuation of the received signals after combining stabilizes, thus improving received signal quality.
Interference Reduction Gain
In mobile cellular communications system, inter-cell interference cannot be ignored due to the frequency sharing and multiplexing nature both within and between cells. However, different from white noise, the interference signal is colored noise. You can combine the expected signals and suppress the interference signal through proper multiantenna spatial weight at the receiving end to improve the average SINR at the receiving end. This is the basis of Interference Reduction Combining feature.
Spatial Multiplexing Gain
The spatial multiplexing gain indicates the improvement of data throughput or transmission rate when the transmit power and bandwidth remain unchanged. You can obtain a spatial multiplexing gain by transmitting multiple parallel data streams over the same time-frequency resources. The spatial multiplexing gain is used to increase system capacity.